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United States Patent |
5,696,449
|
Boskamp
|
December 9, 1997
|
RF coil for open MR magnet
Abstract
A whole body RF coil provided for use with an open MR magnet system
comprises first and second RF coil components selectively spaced apart
from each other on opposite sides of an imaging space disposed to receive
a subject, each of the RF coil components comprising a plurality of
electrically conductive inner and outer arc segments which respectively
lie on corresponding inner and outer circular paths. The RF coil is
constructed to allow the subject to pass through the inner and outer
circular paths of a selected one of the coil components or alternatively
between the spaced apart RF coil components, into imaging space. The first
and second RF coils are each energized in quadrature to generate
respective corresponding first and second magnetic fields which
collectively provide a resultant field comprising the B.sub.1 field
component required for imaging.
Inventors:
|
Boskamp; Eddy B. (Menomonee Falls, WI)
|
Assignee:
|
General Electric Company (Milwaukee, WI)
|
Appl. No.:
|
659109 |
Filed:
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June 3, 1996 |
Current U.S. Class: |
324/318; 600/422 |
Intern'l Class: |
G01V 003/00 |
Field of Search: |
324/318,322,314,307,309
128/653.5,653.2
|
References Cited
U.S. Patent Documents
4680548 | Jul., 1987 | Edelstein et al. | 324/318.
|
4692705 | Sep., 1987 | Hayes | 324/318.
|
4752738 | Jun., 1988 | Patrick et al. | 324/318.
|
5045792 | Sep., 1991 | Mehdizadeh | 324/318.
|
5050605 | Sep., 1991 | Eydelman et al. | 324/318.
|
5365927 | Nov., 1994 | Roemer et al. | 128/653.
|
5473251 | Dec., 1995 | Mori | 324/318.
|
Other References
Circular Polarized RF Coils for Open Magnet Systems. I. Viohl,Ph.D. Book of
Abstracts--3rd Annual Scientific Meeting of Society of Magnetic Resonance.
p. 183 (Nice, France 1985).
|
Primary Examiner: Arana; Louis M.
Attorney, Agent or Firm: Skarsten; James O., Pilarski; John H.
Claims
What is claimed is:
1. An RF coil system for use with an open MR magnet comprising a pair of
main magnet components positioned on opposing sides of an imaging space
disposed to receive at least a portion of a subject for imaging, said RF
coil system comprising:
first and second RF coil components positioned on opposing sides of said
imaging space, said first and second coil components each comprising a
plurality of closed conductive loops, the conductive loops of each of said
RF coil components including inner and outer conductive arc segments, the
inner arc segments of an RF coil component and the outer arc segments
thereof lying along common inner and outer circular paths respectively;
and
means for energizing said closed loops of said first and second RF coil
components to generate respective corresponding first and second magnetic
fields which collectively provide a resultant field comprising a circular
polarized field component in a plane intersecting said imaging space and
substantially parallel to planes respectively containing said inner and
outer circular paths.
2. The RF coil system of claim 1 wherein:
a given one of said closed conductive loops includes a pair of conductive
elements, each of said conductive elements connecting an end of the inner
arc segment of said given closed loop to a corresponding end of the outer
arc segment of said given closed loop, said conductive elements of said
given closed loop being electrically isolated from conductive elements of
other of said closed loops.
3. The RF coil system of claim 2 wherein:
each of said RF coil components has an associated pair of orthogonal
reference axes;
each of said RF coil components comprises a pair of first closed loops
positioned along one of its associated reference axes in spaced apart
opposing relationship, and a pair of second closed bops positioned along
its other associated axis in spaced apart opposing relationship; and
said energizing means comprises means for coupling a first energizing
signal to each of said first closed loops of said RF coil components and a
second energizing signal to each of said second closed loops of each of
said RF coil components, said first and second energizing signals being
substantially the same except that the second signal is delayed in phase
by 90 degrees with respect to said first signal.
4. The RF coil system of claim 3 wherein:
said planes respectively containing inner and outer circular paths of one
of said RF coil components are separated by a specified distance along an
axis orthogonal to said planes.
5. The RF coil system of claim 4 wherein:
each of the inner and outer arc segments of the closed conductive loops of
said RF coil components has an angle on the order of 140 degrees.
6. The RF coil system of claim 1 wherein:
the inner and outer arc segments of a given closed loop are connected
together by a pair of conductive elements extending therebetween, each of
said extending conductive members also interconnecting inner and outer arc
segment of a closed loop adjacent to said given closed loop.
7. The RF coil system of claim 6 wherein:
the inner arc segments of each of said RF coil components are joined
together to form a first closed ring and the outer arc segments of each of
said RF coil components are joined together to form a second closed ring;
and
the first and second closed rings of each of said RF coil components are
joined together by a plurality of said extending conductive elements, said
plurality including first and second conductive elements separated by 90
degrees from each other around their corresponding first and second closed
rings.
8. The RF coil system of claim 7 wherein:
said energizing means comprises means for coupling a first energizing
signal to each of said first conductive elements of said RF coil
components, and a second energizing signal to each of said second
conductive elements of said RF coil components, said first and second
energizing signals being substantially the same except that the second
signal is delayed in phase by 90 degrees with respect to the first signal.
9. An RF coil system for use with an open MR magnet comprising a pair of
main magnet components positioned on opposing sides of an imaging space
disposed to receive a subject for imaging, said RF coil system comprising:
first and second RF coil components selectively spaced apart from each
other on opposite sides of said imaging space, each of said RF coil
components comprising a plurality of electrically conductive inner and
outer arc segments respectively lying on corresponding inner and outer
circular paths, said RF coil components respectively constructed to allow
said subject to pass through the inner and outer circular paths of a
selected one of said RF coil components into said imaging space, and
alternatively to allow said subject to pass between said spaced apart RF
coil components, into said imaging space; and
means for energizing said first and second RF coil components in quadrature
to generate magnetic fields respectively corresponding thereto, said first
and second magnetic fields collectively providing a resultant field
comprising the B.sub.1 magnetic field component in a plane orthogonal to
the direction of the main magnetic field component for MR imaging.
10. The RF coil system of claim 9 wherein:
each of said RF coil components has an associated pair of orthogonal
reference axes; and
each of said RF coil components comprises a pair of first coil loops
positioned along one of its associated reference axes in spaced-apart
opposing relationship, and a pair of second closed loops positioned along
its other associated axis in spaced-apart opposing relationship, each of
said closed loops including one of said inner arc segments and one of said
outer arc segments.
11. The RF coil system of claim 10 wherein:
said energizing means comprises means for coupling a first energizing
signal to each of said first closed loops of said RF coil components and a
second energizing signal to each of said second closed loops of said RF
coil components, said first and second energizing signals being
substantially the same except that the second signal is delayed in phase
by 90 degrees with respect to the first signal.
12. The RF coil system of claim 9 wherein:
the inner and outer arc segments of each of said RF coil components are
respectively joined together to form respective first and second closed
end rings, the first and second end ring of each of said RF coil
components being connected together by a selected number of conductive
elements extending therebetween and positioned in uniformly spaced-apart
relationship.
Description
BACKGROUND OF THE INVENTION
The invention disclosed and claimed herein generally pertains to an RF coil
system for use with an open magnetic resonance (MR) magnet, that is, a
magnet which generates a main magnetic field for MR imaging in a space
which is very accessible to a patient or other imaging subject, and is not
enclosed within a magnet bore. More particularly, the invention pertains
to a full body RF coil system for use in connection with an open MR
magnet.
A very significant innovation in the field of MR imaging and magnetic
resonance therapy (MRT) has been the development of the open MR magnet.
Such magnet, as described for example in commonly assigned U.S. Pat. No.
5,365,927, issued to Roemer et al on Nov. 22, 1994, generally comprises
two donut-shaped ring magnets or magnet components which are positioned on
opposite sides of an imaging space. The two ring magnets are energized to
provide a resultant field in the imaging space which comprises the static
magnetic field for MR imaging. This configuration enables an imaging
subject to enter and leave the space with much more convenience than the
prior art closed bore arrangement. More specifically, the subject can
enter the imaging space either by passing through the center of one of the
ring magnets (front entry) or can enter directly into the space between
them (side entry). Also, the subject can be seated in a chair during the
imaging scan. Moreover, the open magnet arrangement enables attendant
physicians to treat and closely monitor a subject while MR imaging is
taking place.
Open MR magnets currently employ flexible surface coils for the RF
transmit, or excitation, and receive coil. Such surface coils comprise a
combination of single loops which provide linear or circular polarized
fields. However, transmit surface RF coils tend to have comparatively low
homogeneity, particularly in a large field of view. Accordingly, it would
be desirable to configure an RF coil system as a combination of a whole
body transmission coil, to obtain high transmit uniformity, and surface
coils for use as RF receive coils, to provide an arrangement having
comparatively high excitation homogeneity in combination with large field
of view. However, any such RF coil arrangement must avoid placing
conductors across the path of either forward or side entry, so that
accessibility into the imaging space will be maintained. Also, in addition
to high homogeneity, the whole body RF coil must be able to provide a
circular polarized field in the transverse plane, i.e., the plane which is
generally orthogonal to the axis of a DC magnetic field extending between
the two ring components of the open magnet.
Moreover, such arrangement must not exceed FDA specified absorption ratio
(SAR) limits. As is known to those of skill in the art, SAR pertains to
the level of electromagnetic energy which can be absorbed by a patient or
medical personnel positioned in or close to the transmit RF coil of an MR
system.
SUMMARY OF THE INVENTION
The invention is directed to an RF coil system for use with an open MR
magnet comprising a pair of main magnet components positioned on opposing
sides of an imaging space disposed to receive at least a portion of a
subject for imaging. The RF coil system comprises first and second RF coil
halves or components likewise positioned on opposing sides of the imaging
space, each of the RF coil components comprising a plurality of conductive
closed loops. The closed loops of each of the RF coil components include
inner and outer conductive arc segments, respectively lying in planes
which are substantially parallel to one other. The RF coil system further
comprises means for energizing the closed loops of the first and second RF
coil components to generate respective corresponding first and second
magnetic fields which collectively provide a resultant field comprising a
circular polarized field. The circular polarized field lies in a plane
which intersects the imaging space and is substantially parallel to the
planes respectively containing the inner and outer arc segments.
Preferably, the planes associated with each RF coil component are
separated from each other by a specified distance along an axis orthogonal
to the planes.
In one embodiment of the invention, which has been found to be particularly
useful in increasing sensitivity, the inner and outer arc segments of a
given one of the closed conductive loops are connected together by a pair
of conductive elements extending therebetween to form the given conductive
loop, wherein the extending conductive elements are electrically isolated
from conductive elements of other closed loops. Each of the RF coil
components has an associated pair of orthogonal reference axes, and
comprises a pair of first closed loops positioned along one of its
associated axes, in spaced apart opposing relationship, and a pair of
second closed loops positioned along its other associated axis in spaced
apart opposing relationship. The energizing means comprises means for
coupling a first energizing signal to each of the first closed loops of
the RF coils and a second energizing signal to each of second closed loops
of the coil. The first and second energizing signals are substantially the
same, except that they are 90 degrees out of phase with respect to one
another. Thus, the first and second loops are driven in quadrature, the
first energizing signal being commonly referred to as an I-channel signal,
and the second energizing signal being commonly referred to as a Q-channel
signal.
In a second embodiment of the invention, which has been found useful in
improving homogeneity, the inner and outer arc segments of a given one of
the closed conductive loops are connected together by a pair of conductive
elements extending therebetween in like manner with the first embodiment.
In this embodiment, however, each of the extending conductive members also
serves to interconnect the inner and outer arc segments of a closed
conductive loop which is adjacent to the given closed loop. The second
embodiment usefully comprises a modification of an RF coil known in the
art as a bird cage coil, as shown, for example, in commonly assigned U.S.
Pat. No. 4,692,705, issued Sep. 8, 1987 to C. E. Hayes. Such embodiment is
likewise driven in quadrature.
It is seen that the RF coil of the invention comprises a set of coil
components or coil halves, positioned in spaced apart relationship on
opposing sides of the imaging space, and each proximate to a corresponding
one of the two magnet poles or magnet components of an open magnet system.
Thus, the RF coil system does not inhibit side entry into the open magnet,
since a patient can be easily moved through the space between the RF coil
components. At the same time, the inner arc segments of each RF component
are positioned along a common circular path. The radius of such circular
path is made large enough to allow front entry into the open magnet, i.e.,
to allow a patient to pass through the circular path into the imaging
space. Thus, the RF coil of the invention readily permits both front and
side entry into the open magnet without any need to change or readjust the
RF coil components.
Moreover, the resultant field produced by the RF coil components,
comprising the RF B.sub.1 field, is contributed by the fields of both RF
coil components. The field B.sub.1 is thereby made strong enough to
provide a comparatively high level of sensitivity.
OBJECTS OF THE INVENTION
An object of the invention is to provide a whole body RF excitation coil
for use in an MR imaging arrangement which employs an open magnet.
Another object is to provide an RF coil system for use, in connection with
an open MR magnet, which achieves significantly improved homogeneity in
combination with a large field of view.
Another object is to provide an RF coil of the above type which does not
diminish accessibility or freedom of movement of a patient or imaging
subject with respect to the imaging space of the open MR magnet.
Another object is to provide an RF coil of the above type having two
components on opposing sides of the imaging space, without requiring any
conductors or other structure connecting them which could block or impede
either front or side entry into the imaging space.
These and other objects of the invention will become more readily apparent
from the ensuing specification, taken together with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified perspective view showing an embodiment of the
invention, wherein certain components thereof are spaced apart from one
another for purposes of illustration.
FIG. 2 is a schematic diagram depicting the I-channel referred to above for
the embodiment of FIG. 1.
FIG. 3 is a simplified diagram depicting an RF excitation/reception circuit
for use with the embodiment of FIG. 1.
FIG. 4 shows a 90 degree delay device for the circuit shown in FIG. 4.
FIG. 5 is a plot of current versus the projection of the RF magnetic field
in the X-Y plane provided by the embodiment of FIG. 1.
FIG. 6 is a perspective view showing an MR open magnet configuration
incorporating the embodiment of FIG. 1.
FIG. 7 is a sectional view taken along lines 7--7 of FIG. 6 showing
respective field components.
FIG. 8 is a perspective view showing another embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, there is shown an RF coil system 10, constructed in
accordance with the invention, for use with an open MR magnet such as
described hereinafter in connection with FIG. 6. Coil system or antenna 10
generally comprises RF coil components 10a and 10b, which are spaced apart
from one another along the Z-axis of the open magnet system on opposing
sides of the imaging space 20 thereof. RF coil 10a comprises two I-channel
conductive loops 12 in spaced apart relationship with each other along an
axis X' parallel to the X-axis of the open magnet system, and further
comprises two Q-channel loops 14 in spaced apart relationship with each
other along an axis Y' parallel to the Y-axis of the magnet system.
Similarly, RF coil 10b comprises two I-channel conductive loops 16 in
spaced apart relationship with each other along an axis X" parallel to the
X-axis, and two Q-channel conductive loops 18 in spaced apart relationship
with each other along an axis Y" parallel to the Y-axis.
Each of the closed loops 12 comprises an electrically conductive inner arc
segment 12a and a conductive outer arc segment 12b, wherein each end of an
inner arc segment 12a is connected to a corresponding end of an outer arc
segment 12b by means of a conductive element 12c. In like manner, FIG. 1
shows each of the closed loops 14, 16, and 18 respectively comprising
inner arc segments 14a, 16a, and 18a, outer arc segments 14b, 16b, and
18b, and conductive elements 14c, 16c, and 18c.
Referring further to FIG. 1, the inner arc segments 12a are shown to lie
along a common circular path 22, while the inner arc segments 14a are
shown to be spaced apart from the path 22. However, it is to be understood
that such spacing is shown only for the purpose of illustrating each of
the arc segments 12a and 14a in its entirety. In fact, each of the inner
arc segments 14a also substantially lies on circular path 22, i.e.,
segments 14a lie as closely along circular path 22 as permitted by the
particular cross-sectional dimensions of the inner arc segments 12a and
14a.
While the angle .theta..sub.i selected for each of the inner arc segments
12a and 14a may vary for different embodiments of the invention, it has
been found that a very useful value of .theta..sub.i is on the order of
140 degrees. Moreover, it has been found advantageous to center the two
inner arc segments 12a along an axis X', and to center inner arc segments
14a along axis Y'. Thus, there is some overlap along the circular path 22
between inner arc segments 12a and 14a. The plane of circular path 22 is
parallel to the X-Y plane of the MR system reference axes, and the Z-axis
passes through the geometric center of the circular path 22.
In like manner, each of the outer arc segments 12b and 14b substantially
lie along a circular path 24, each of the inner arc segments 16a and 18a
lie along a circular path 26, and each of the outer arc segments 16b and
18b lie along circular path 28. The Z-axis likewise passes through the
respective geometric centers of circular paths 24, 26, and 28, such paths
respectively lying in planes parallel to the X-Y plane. The angle of each
of the arc segments 12b and 14b, 16a and 18a, and 16b and 18b, preferably
is the same as the angle of inner arc segments 12a and 14a, that is, 140
degrees. The radius r.sub.i of respective inner arc segments has usefully
been found to be 30 centimeters, and the radius r.sub.o of respective
outer arc segments has usefully been found to be 50 centimeters. A radius
of 30 centimeters for r.sub.i is sufficiently large to enable a patient to
be moved through the circular path 22 or 26, as described hereinafter.
However, the invention is not limited to such values. It is to be noted
further that the arc segments 12b and 14b are centered along the X' and Y'
axes, respectively. Arc segments 16a and 16b are centered along axis X",
and segments 18a and 18b are centered along axis Y".
It has been found that homogeneity of the resultant magnetic field produced
by operation of RF coil components 10a and 10b, as hereinafter described,
can be significantly improved by offsetting the plane of circular path 22,
containing inner arc segments 12a and 14a, outwardly along the Z-axis
relative to the plane of circular path 24, containing outer arc segments
12b and 14b. Each of the conductive elements 12c and 14c is provided with
a 90 degree bend to achieve this. Preferably, the displacement between the
two planes is on the order of 7.5-10.0 centimeters although the invention
is by no means limited thereto. Similarly, the plane containing inner arc
segments 16a and 18a is offset outwardly along the Z-axis, i.e., away from
imaging space 20 in the opposite direction by a like displacement, with
respect to the plane containing outer arc segments 16b and 18b.
FIG. 1 further shows the direction of current flow through respective loops
12, 14, and 18 when coils 10a and 10b are energized. As stated herein, the
loops are energized in quadrature, that is, the same signals applied to
loops 12 and 16 are also applied to loops 14 and 18, respectively, but
delayed by ninety degrees in phase therefrom. The ninety degree phase
delay is depicted in FIG. 1 by employing double-headed arrows to show
current flow around loops 14 and 18.
Referring to FIG. 2, there are shown I-channel loops 12 and 16, of RF coil
components 10a and 10b, respectively, coupled to receive an RF excitation
signal e.sub.I through coaxial cable 30 and a matching network 32. The two
loops 12 and the two loops 16 are interconnected by means of conductive
paths 12d and 16d, respectively, so that current respectively flows
therearound as indicated in FIG. 2. Current flow through respective arc
segments of the I-channel loops causes corresponding magnetic field
components to be generated in the imaging space 20. These field components
will substantially cancel each other outside the X-Y or transverse plane,
but will be additive therein to provide a circularly polarized B.sub.1
field.
FIG. 2 further shows components of the closed conductive loops 12 and 16
provided with respective capacitive elements 12e-h and 14e-h, as
respectively shown. The capacitances provided by such capacitive elements,
together with the inductance of the respective loops, provide the RF
system with a selected resonant frequency. For the embodiment, the
resonant frequency is usefully selected to be on the order of 21 MHz,
although other frequencies may alteratively be used.
Referring further to FIG. 2, there is shown network 32 comprising
capacitors 32a-32d, capacitors 32b and 32c being adjustable. While not
shown in FIG. 2, it is to be understood that the Q-channel loops 14 and 18
are each energized by an excitation signal e.sub.Q, which is identical to
signal e.sub.I except that it is delayed by 90 degrees in phase therefrom.
Referring to FIG. 3, there is shown a conventional transmit/receive circuit
34 for an MR system. Circuit 34 includes an RF transmit amplifier 36, a
receive amplifier 38, a transmit/receive (T/R) switch 40 and a delay
device 42 for providing respective I- and Q-channel excitation signals.
During an MR pulse sequence, transmit amplifier 36 is operated to generate
the RF signal e.sub.T. Synchronously therewith, the T/R switch 40 is
operated to couple the RF excitation signal to the delay device 42. Device
42 passes the signal e.sub.T to the I-channel as the excitation signal
e.sub.I while delaying the signal e.sub.T by 90 degrees to provide the
Q-channel excitation signal e.sub.Q.
During the acquisition phase of the MR pulse sequence, received MR signal
components e'.sub.I and e'.sub.Q are respectively detected by the
I-channel and Q-channel loops of the RF coils, and coupled back to device
42. Device 42 operates on e'.sub.I and e'.sub.Q to produce a resultant
received MR signal e.sub.R therefrom, which is coupled to T/R switch 40.
During such acquisition phase, switch 40 is operated to couple the
received MR signal to receive amplifier 38, which amplifies such signal
for further processing in accordance with well known MR techniques.
Delay device 42 comprises a conventional device, one example of which is
shown in FIG. 4. Capacitive elements 44-54 are respectively connected to
inductive elements 56 and 58, as shown.
When the respective conductive loops of RF coil components 10a and 10b are
driven by the I- and Q-channel excitation signals, the magnetic field
components generated thereby in imaging space 20 are additive therein to
provide a resultant circular polarized magnetic field. Such magnetic field
comprises the B.sub.1 field, and flips the spins in an MR patient or other
subject from alignment with the Z-axis into the transverse X-Y plane, in
accordance with MR practice. Referring to FIG. 5, there is shown a plot of
such resultant magnetic field in the X-Y plane for Z=0.0. Such plot shows
the X-Y projection of the magnetic field provided by RF coil system 10 to
have very high homogeneity.
Referring to FIG. 6, there is shown an open magnet system 60, comprising
ring magnet poles 60a and 60b in spaced apart relationship on opposing
sides of imaging space 20. Magnets 60a and 60b comprise permanent,
superconducting or other type of magnet for producing a static B.sub.0
magnetic field, as required for MR imaging, the B.sub.0 field being
directed along the z-axis. Magnets 60a and 60b are respectively provided
with bores or circular apertures 62a and 62b formed therethrough. A
patient or other subject (not shown) may enter the imaging space by
passing through one of the bores such as bore 62b (front entry) and be
supported for imaging on a couch 64 or the like. Alteratively, the subject
can enter into imaging space 20 between the magnets 60a and 60b from along
the direction of arrow 66 (side entry). Referring further to FIG. 6, there
are shown gradient coil assemblies 68a and 68b respectively positioned
within ring magnets 60a and 60b around the bores 62a and 62b. Each
gradient assembly comprises a set of concentric shielded X-, Y-, and
Z-gradients for generating respective X-, Y-, and Z-gradient field
components in imaging space 20. Such gradient coil arrangements are well
known in the art and are accordingly not shown in detail.
FIG. 6 further shows RF coil components 10a and 10b respectively positioned
in adjacent relationship with magnets 60a and 60b, and enclosed in
coverings or shrouds 70a and 70b, respectively. Each RF coil component 10a
and 10b is supported on a coil form (not shown) so that the inner arc
segments 12a and 14a of component 10a are inserted into bore 62a of magnet
60a, and the inner arc segments 16a and 18a of component 10b are inserted
into bore 62b of magnet 60b. Respective electrical transmission paths to
RF coil components 10a and 10b, such as through cable 30 and network 32 as
described above in connection with FIG. 2, can be contained within upper
support member 72a or lower support member 72b, which collectively assist
in holding magnets 66a and 66b and RF coil components 10a and 10b in
place. Thus, such transmission paths do not block or impede side access
into the imaging space 20. Also, the minimum diameter of coil components
10a and 10b, which is determined by the diameters of circular paths 22 and
24, is sufficiently large to allow front entry into imaging space 20.
Referring to FIG. 7, there are shown RF field components F.sub.a and
F.sub.b, which are respectively produced when RF coil components 10a and
10b are energized by the signal e.sub.I as described above. Portions or
components of fields F.sub.a and F.sub.b collectively add together, in the
imaging space, to provide a resultant field comprising the B.sub.1 field.
e.sub.Q generates a similar pattern, geometrically rotated around the
Z-axis by 90 degrees, to produce F'.sub.a and F'.sub.b which also combine
together. In fact, by providing contributions to the total B.sub.1 field
from both the RF coil components 10a and 10b, the amplitude of B.sub.1
provides a higher level of sensitivity and uniformity then would be
provided by a coil on only one side of imaging space. As is well-known to
those of skill in the art, sensitivity in MR is defined as B.sub.1 divided
by the level of the current flowing in the coil used to produce the
B.sub.1 field.
Referring further to FIG. 7, there is shown a shield 74a positioned between
RF coil component 10a and gradient coil assembly 68a. There is further
shown a shield 74b positioned between RF coil component 10b and gradient
coil assembly 68b. The shields 74a and 74b serve to minimize RF losses in
the gradient coils.
Referring to FIG. 8, there is shown a second embodiment 76 of the RF coil
of the invention, which comprises RF coil components 76a and 76b.
Component 76a comprises conductive end rings 78a and 80a, which both lie
in planes parallel to the X-Y plane, and are centered along the Z-axis.
Ring 76a has a diameter which is selectively less than the diameter of end
ring 80a, and the two end rings are joined by conductive elements 82,
which are positioned therearound at regular intervals. The number of
conductive elements 82 is preferably divisible by four, such as twelve
elements spaced at thirty degree intervals. Preferably also, each
conductive element 82 contains a 90 degree angle bend, so that end ring
78a is spaced apart from end ring 80a along the Z-axis, such as by a
displacement of 7.5-10.0 centimeters. Thus, two adjacent conductive
elements 82 and the segments of end rings 78a and 80a positioned
therebetween together comprise a closed conductive loop.
Similarly, RF coil component 76b comprises conductive end rings 78b and
80b, likewise lying in planes parallel to the X-Y plane and centered along
the Z-axis. The diameter of end ring 78b is selectively less than the
diameter of end ring 80b, and the two end rings are connected by
conductive elements 84 which are similar or identical in form and number
to the conductive elements 82.
It will be apparent to one of skill in the art that the RF coil components
76a and 76b comprise a variation of the bird cage RF coil referred to
above. Such coils may be thought of electrically as comprising a delay
line of one wavelength wrapped around a cylinder or other closed path, and
connected to itself. Such coils are energized by applying a source of RF
power across a capacitive element, in one of the conductors connecting the
end rings of the bird cage coil together.
Accordingly, to energize components 76a and 76b, a matching network 86,
similar to network 32 described above, is connected to capacitor 88
positioned in conductive element 82a of RF coil component 76a, and also
across capacitor 90 positioned in conductive element 84a of RF coil
component 76b. The I-channel excitation signal e.sub.I is coupled to coil
components 76a and 76b from coaxial cable 30 through network 86, which
comprises capacitors 86a-d. Moreover, to drive RF coil arrangement 76 in
quadrature, a second matching network 92, comprising capacitors 92a-d, is
coupled across capacitors 94 and 96, respectively positioned in conductive
element 82b of coil component 76a, and conductive element 84b of coil
component 76b. It is to be noted that elements 82b and 84b are positioned
90 degrees around their respective coil components from conductive
elements 82a and 84a. The Q-channel excitation signal e.sub.Q is coupled
to RF coil components 76a and 76b through network 92, from a coaxial cable
98. Each of the other conductive elements 82 and 84 likewise includes a
capacitive element (not shown).
When RF coil components 76a and 76b are respectively excited as described
above, they produce magnetic field components in the imaging space
respectively similar to field components F.sub.a and F.sub.b shown in FIG.
7. Thus, the RF coil 76 produces a resultant circular polarized field in
the X-Y plane, comprising the B.sub.1 field, in like manner with RF coil
10. Also, RF coil components 76a and 76b may be positioned with respect to
open magnets 62a and 62b, respectively, in like manner as RF coil
components 10a and 10b, as shown in FIG. 6. The dimensions of end rings
78a and 78b are sufficiently large to allow front entry therethrough into
imaging space 20, and electrical transmission paths between coil
components 76a and 76b can be contained in upper and/or lower support
members 72a and 72b, allowing side entry.
Obviously many other modifications and variations of the present invention
are possible in light of the above teachings. It is therefore to be
understood that within the scope of the disclosed concept, the invention
may be practiced otherwise than as specifically described.
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